1. Technical Field
The present invention relates to a jitter measurement apparatus, a jitter calculator, a jitter measurement method, a recording medium, a communication system, and a test apparatus.
2. Related Art
Conventional methods for measuring timing jitter with a digital comparator include an undersampling method using a comparator and a fail counting method. These methods measure the RMS value of the jitter in the signal under measurement by calculating a jitter histogram.
The undersampling method using a comparator compares voltage of a signal under test to a reference voltage at strobe timings having frequencies no greater than the Nyquist frequency and supplied from a timing generator. For example, the test apparatus uses, as a repeating strobe signal, a trigger signal generated from the starting time of the test cycle in synchronization with the test signal. Here, strobes having phases that gradually change relative to the test signal can be generated by increasing the time shift between strobes in each test cycle, and these strobes can be used to sample the test signal.
The ratio of logic H at each strobe position is then calculated based on the data obtained from the sampling, thereby obtaining a cumulative density function (CDF) and a probability density function (PDF) of the transition edge timing. The undersampling method using a comparator measures the jitter in this way, as shown, for example, in W. Dalal, and D. Rosenthal, “Measuring Jitter of High Speed Data Channels Using Undersampling Techniques,” in Proc. IEEE International Test Conference, pp. 814-818, Washington, D.C., Oct. 18-23, 1998.
The fail counting method measures the jitter by comparing the logic value of the signal under test to an expected value at a timing designated by a strobe. The CDF of the edge timing can be obtained by a fail counter, as shown in, for example, Y. Cai, L. Fang, R. Ratemo, J Liu, K. Gross, and M. Kozma, “A Test Case for 3 Gbps Serial Attached SCSI (SAS),” in Proc. IEEE International Test Conference, Austin, Tex., Nov. 8-10, 2005.
These timing jitter measurement methods, however, obtain the CDF by calculating the ratio of logic H at each strobe timing while gradually increasing the amount of time-shift of the strobe. Such methods take time to sample the amount of data used for the jitter analysis. A jitter measurement method and a jitter measurement apparatus are desired that can accurately calculate the jitter with a short measurement time.